1. Field of the Invention
This invention relates to systems and methods for processing optical data, and more particular to the amplification and retroreflection of an optical information bearing beam.
2. Description of the Related Art
Optical amplification and retroreflection are two functions that are used in various stages in the processing of optical data. The two functions are incorporated together in an optical associative memory for pattern recognition, such as the systems disclosed in U.S. Pat. Nos. 4,739,496 to Marom, et al., "Associative Holographic Memory Apparatus Employing Phase Conjugate Mirrors", and 4,750,153 to Owechko, et al., "Associative Holographic Memory Apparatus Employing Phase Conjugate Mirrors and a Two-Wave Mixing Contra-Directional Coherent Image Amplifier", both assigned to Hughes Aircraft Company, the assignee of the present invention. Associative memories are also discussed in Dunning, et al., "All-Optical Associative Memory with Shift Invariance and Multiple-Image Recall", Optics Letters, Vol. 12, No. 5, May 1987, pages 346-348, and Soffer, et al., "Associative Holographic Memory with Feedback Using Phase-Conjugate Mirrors", Optics Letters, Vol. 11, No. 2, February, 1986, pages 118-120. Amplification and retroreflection functions are also combined in optical computers to amplify and redirect a two-directional array of data.
Various devices for optical amplification and/or retroreflection use photorefractive materials. In such materials, the index of refraction changes under the influence of applied light, such as a laser beam. The light causes charges within the photorefractive material to migrate and separate, producing an internal space charge electrostatic field. This field produces a change in the crystal's refractive index by the linear electro-optic effect (the Pockels effect). Photorefractive materials generally comprise BaTiO.sub.3, Bi.sub.12 SiO.sub.20, KTa.sub.1-x Nb.sub.x O.sub.3, BGO and LiNbO.sub.3, and III-V and II-VI semiconductor materials within the periodic table, such as, for example, GaAs and InP.
Optical amplification, without retroreflection, has been accomplished by two-wave mixing in a photorefractive crystal. An information bearing probe beam intersects a pump beam within the crystal. With a proper crystal orientation relative to the two beams, energy is transferred from the pump to the probe beam to amplify the latter. While ideally the probe beam would simply be amplified without any other change, in practice the photorefractive crystal will frequently impose unwanted additional phase information upon the probe beam, thus distorting its information content. Two-wave mixing within photorefractive materials is discussed in Laeri, et al., "Coherent CW Image Amplifier and Oscillator Using Two-Wave Interaction in a BaTiO.sub.3 -Crystal", Optics Communications, Vol. 47, No. 6, Oct. 15, 1983, pages 387-390, and also in Feinberg, et al., "Photorefractive Effects and Light Induced Charge Migration in Barium Titanate", Journal of Applied Physics, Vol. 51, No. 3, March, 1980, pages 1297-1305.
High gain laser media, such as metal vapors or dyes, have also been used for optical amplification. However, due to the spontaneous decay of the excited states, these systems are noisy and require an intense input to have a useful signal-to-noise ratio. This often leads to pulsed amplification, as opposed to the two-wave mixing technique which can operate continuously at flux intensities on the order of milliwatts per cm.sup.2.
Turning now to retroreflection, a special form of retroreflection referred to as phase conjugation has been achieved with devices known as phase conjugate mirrors (PCMs). A PCM produces a retroreflection of an incident beam, with the phase of the reflected beam reversed from that of the incident beam at the point of reflection. Several different ways of producing phase conjugated beams have been discussed in the literature, including four-wave mixing and self-pumped mechanisms. The theory and operation of PCMs, along with an explanation of the photorefractive effect, is described in a chapter by Feinberg, "Optical Phase Conjugation in Photorefractive Materials", within the text "Optical Phase Conjugation", ed. by Fisher, Academic Press, Inc., 1983, pages 417-443. Of the self-pumped PCMs, those employing Brillouin or Raman scattering are generally used with high power pulsed laser beams, such as from a Nd:YAG laser, but are not practical with low power continuously operated lasers such as HeNe or low flux Argon ion laser devices. Another type of self-pumped PCM uses a photorefractive material with a high electro-optical coefficient as the phase conjugating medium. This type of self-pumped PCM has been employed with continuously operating, low-power lasers, but tends to produce a somewhat noisy conjugated output. After a long buildup time, the self-pumped PCMs emit a conjugated output, but do not provide amplification.
Another class of devices that perform retroreflection but not amplification is referred to as "pseudoconjugators". These devices resemble PCMs in that they retroreflect incident beams. However, they do not perform a true conjugation or wavefront reversal on the incident wavefront. Pseudoconjugators have commonly been configured as arrays of corner reflectors. Such devices are discussed in articles by O'Meara, "Wavefront Compensation with Pseudoconjugation", Optical Engineering, Vol. 21, No. 2, March/April 1982, pages 271-280, and in Jacobs, "Experiments with Retrodirective Arrays", Optical Engineering, Vol. 21, No. 2, March/April 1982, pages 281-283. In the O'Meara article, the use of a pseudoconjugator in a double-pass reflective conjugation compensation system is illustrated in FIG. 2(a). In such a system, an input beam is initially transmitted through a distorting medium, and then retroreflected by a pseudoconjugator back through the distorting medium to remove the distortions.
While pseudoconjugators do achieve a retroreflection function, they are non-amplifying. Also, they include no inherent mechanism to compensate for noise in an input beam.
A combination of both amplification and retroreflection has been achieved with the four-wave mixer PCM. Such a device is illustrated in simplified form in FIG. 1. A pair of contradirectional laser beams 2 and 4 are directed into a photorefractive mixing medium 6. An initializing laser beam E.sub.i, equal in frequency to beams 2 and 4, is directed into the mixing medium from the side. As a result of the action of the various beams within the mixing medium, a reflected beam RE.sub.i *, where R is the coefficient of reflectivity, is reflected back in a direction opposite to incident beam E.sub.i. Since power is pumped into the system by beams 2 and 4, the PCM may be configured to provide an amplification which makes R greater than 1.
A system for implementing a four-wave mixer is illustrated in FIG. 2. An input beam 8 is divided by a beam splitter 10 into first and second beams 12 and 13. Beam 12 passes through an optical mask 14, from which it acquires information. A portion of the beam is diverted for input monitoring by a beam splitter 15, while the remainder is focused by a lens 16 as the probe beam E.sub.i. The second beam 13 is reflected off a mirror 18 to a beam splitter 20, from which it emerges as pump beams 2 and 4. These beams are reflected off mirrors 22 and 24, respectively, and directed into the photorefractive mixing medium 6. The conjugated return beam is deflected by beam splitter 15 to yield an output beam 25.
The system of FIG. 2 is fairly complex to set up, since all of the beams must interact within the same volume of the non-linear photorefractive medium 6, and careful alignment is required. While the system is illustrated for only a single beam at a single frequency, in practice beams with multiple frequencies may be used. Since pump beams must be provided with the same frequency as the probe beam for each different frequency, additional equipment and alignment complexity may be required.
Another limitation of the four-wave mixing approach is encountered when a very rapid system response is desired. In this situation, a four-wave mixing system will often use sodium vapor as the non-linear medium. However, sodium vapor requires a very small angle, on the order of one-half to two degrees, between the pump and probe beams. Since some amount of spreading is normally associated with the probe beam which bears complex spatial information, this small angle requirement can significantly restrict the probe beam's information carrying capability.